Electrostatic loud-speaker



March 10, 1953 A. A. JANSZEN ELECTROSTATIC LQUD SPEAKER 3 Sheets-Sheet 1Filed Oct. 5, 1949 Fig.l

March 10, 1953 A. A. JANSZEN 2,631,196

ELECTROSTATIC mun SPEAKE Filed 001:. 5, 1949 s Sheets-Sheet s SPEAKER,yg

INPUT 6 TERMINALS (F =F T "1 Fig.7

T0 44 STATIONARY ELECTRODE SPEAKER INPUT TERMINALS 6 8 a T T 2 1 INPUTTERMINALS i l l l l.lu l.l ll l 50 lgVENTO/i Patented Mar. 10, 1953UNITED STATES PATENT OFFICE 2,631,196 ELECTROSTATIC LOUD- SPEAKER ArthurA. J anszen, Cambridge, Mass.

Application October 5, 1949, Serial N 0. 119,639

The present invention relates to electroacoustical transducers and moreparticularly to loudspeakers.

Because of the inherent simplicity of a system in which a driving forceis essentially constant over the area which is to vibrate (in contrastto the application of force at or near the apex of a conical structureused as a diaphragm in the conventional electrodynamic loudspeaker),many attempts have been made in the past to develop aloudspeaker'utilizing the electrostatic principle, in which theelectrostatic forces existing between essentially parallel surfaces of acondenser are used to set one of the surfaces in motion. So far as Iknow, however, no electrostatic loudspeaker having adequate protectionagainst electrical breakdown and having satisfactory frequency response,power output, and polar energy distribution characteristics, has beendeveloped. Bass response has been poor because of the use of diaphragmmaterials which have low compliance, the use of small vibrating segmentsinto which the total vibrating surface was broken up, and the presenceof trapped air between the vibrating surface and the stationary surface.This trapped air constituted a capacitive reactance which added to theeffective stiffness of the vibrating system. High frequency response(greater than 5,000 C. P. S.) has been poor because of the high mass perunit area of the moving surface, and the difiiculty of efiecting anefficient coupling to ordinary power amplifiers at high frequenciesbecause of the fact that the impedance looking into the loudspeakers hasbeen essentially the capacitive reactance of a condenser.

Furthermore, when diaphragm areas sufiicient for the production of highacoustic. energy levels at low frequencies were used, the polardistribution of energy at the high frequencies was unsatisfactorybecause of the highly directive properties of flat radiators atfrequencies where their circumference is many times the wavelength; inother words, the speakers were too directive at high frequencies.

The principal object of the present invention is to obtain a highacoustic power output per unit area over the wide range ofaudio-frequencies necessary for effective loudspeaker operation.

With this object in view, one of the principal features of the presentinvention comprises an" acoustically transparent stationary-or backelectrode which improves the high frequency response by allowing thediaphragm to vibrate at the driving frequency without hindrance due to 7Claims. (Cl. 179-111) entrapment of air. In one form of the inventionthe back electrode comprises a mesh or zig-zag of Wire in which thesuccessive lengths are sufficiently close together to provide asubstantially uniform electrical field at the diaphragm. In view of thehigh polarizing and driving potentials necessarily used in anelectrostatic speaker, the wires are preferably coated with a highdielectric material in order that the diaphragm may not have to assumeany insulating function and may be as thin as possible.

Another feature of the invention resides in a construction whereby oneor both of the electrodes constituting the condenser of the transducerare provided with insulation of such character that the air gap canbeoperated at or near its ionization potential. While the use of soliddielectric materials in loudspeakers and microphones is not new, it hasbeen used mainly to provide points of support for the diaphragm. So faras I am aware, it has not previously been proposed to operate the airgap at or near ionization potential. As will be pointed out in thespecific description of the apparatus according to the presentinvention, the realization of this feature requres the use of adielectric material having certain qualities, namely, high dielectricstrength, high dielectric constant, and an appropriate leakageresistance. Various materials may be used for this purpose, but apolyvinyl chloride dielectric has been found satisfactory in practice.

A further feature consists in electrically dividing the diaphragm intorelatively small areas which are separately excited ,throughnet'works ofsuch character that a'desirable frequencyresponse characteristic, polarenergy distribution characteristic, and impedance characteristic canbe'obtained.

Other features of the invention consist of oertain novel features ofconstruction and combinations and arrangements of partshereinafterdescribed and .particularly defined in the claims.v

In the accompanying drawings, Fig. 1 is a front elevation of one form ofloudspeaker according to the present invention; Fig. 2 is a sectionaldetail view of the electrodes; Fig. 3 is an enlarged detail view showingthe spacers;'

Fig. 415 a plot of the impedance characteristics; and Figs. 5 and 6 arediagrams of equivalent networks of the speaker; Fig. '7 is an elevationof a modified form of the invention; Fig. 8 is av diagram of anotherequivalent network; and Fig. 9 shows another modified form of theinvention. r

The loudspeaker according to the preferred a 3 form of the presentinvention as shown in the drawings comprises a back electrode indicatedgenerally at 6 and a front electrode or diaphragm 8. The back electrodeis in the form of parallel stretches of wire threaded back and forthbetween pins 9. The individual lengths of wire are insulated with adielectric material indicated at H] in Fig. 2. While the individuallengths or passes of wire are close together, there is sufficient spacebetween them to prevent entrapment of air and the back electrode maytherefore be considered as acoustically transparent. The lengths; ofwire are so mounted that the entire back elece. trode has only a smallinductance and the entire electrode assumes practically a uniioifm llqlential, throughout its entire area at all frequencies within the audiorange.

The front electrode comprises a diaphragm; preferably of rubber chloride(Pliofilm) in a very she t, wh ch. m y b b t; .0 04 h k: 1.65 Theelectrode is mounted in a suitable. frame and Stretched taut at a smalldistance in, front of the back electrode 6. Thediaphragm sv sp c d.from. th back e e r de byme ns of we ten s-sh wn as pie s of di l c c aerial. Th spacers a e-about -04; inch in e ght. and a e ir g lar ydistrib ted v r the sur ce of. the electrodes whereby the diaphragm isme-. chamcel y iyidedintoir s ar a s d s g e n. a. generalway bythenumerals 14, t etc. Fig. 3,. Theseareas. are of non-uniform size andconfigu a ons.- a d, non-symm trical wh reby; h ir n tur i equencies dner sli h ly rom-cam v anothe dienh eem. is Pro ide h a csndu ns.coating, preferably in the nature of a dag coating n. w i hi the geleizi g and r in p e t al-s. are BR iQ- As. w ll c de-s bed. ater. heeqetne s. rzreIe h y pplied. n. a. manner o; divi e th iaphragm. ele tically n o. areas hayi g. ect e re pons s; in dif erent requ nc ban pat. iron; this; featu e howev r. the. transducer; s shown in n a eedsectin nEie- 72.5

n F g: sev re, re ches f the rire 6; or. t e: ccnductina baekn a areshown... ea hw reb ns.

' ith.asheath-v Q: of.

hitheff r, re s ns t9. h rexnla edlateLis pref:-

m lxe t elw .%9.IL ma be, m de-h pre erred dimensipna. namely: wirediameter 0.015"; diameter-ofsheath. -04.07 air n 4.1%! 21l li!,ess:ofdiaphragm 0 ,0o04

e; neratien Qt.- the peaker he principal:

e1 ises item the. fact, that. the dielec: .i ,te l, l fish s an:appreciable. but not. infie, ta, leaka e resistance- This; is. indicatedby. the equivalent circuit of the loudspeaker which.

sh wn; n; 51s.. 5; which:

Cd is the capacity between the two surfaces of. he d hr G is. h ci y.betw en he nner s rf e of the, diaphra m a d... he ut r su face f hepqlyvi yl h eri e sheath. 1- t e ai sea capacit 5. s, he an i st betweene stationar el ctmde conductor and the outer surface ofv the polyvinyhloride. e, andvRea e the r spectiv z e a eres sts; ances cr sshe h e ancitan s,

es 1 2mm Ep is the polarizing voltage, t. the cu rent. th l9ulsnee ranectric material;

1' hloride. While exact dimene. inven ion.

he seri s. esistance is lating he-p larizin Iac is the A. C. current inthe loudspeaker due to the signal voltage appearing across the secondarywinding of the output transformer of the power amplifier.

Typical values of Cd, 02., Cs, Re, Rs, and Ip which have been obtainedexperimentally are noted in Fig. 5. The values of the Us are such thatpractically all .2%). of the A, C si nal voltage appears; acros the airgap when the dimensions are those given in Fig. 5. The distribution ofthe D. C. polarizing voltage, however, is not determined by the relativevalues of the Us, but by the-relative values of the Rs, once a steadystate condition exists, And the relative values of these Rs change sinceRe. is not constant, but is infihfifld @iHIiQtiQHQi-j the potentialexisting across it, decreasing as the potential increases. If thepolyvinyl chloride were a perfect insulator, i. e., if its leakageresistance were infinite, no D. C. polarizing potentialcould existacrosstheair gap, since the surface of the poly-vinyl chloride would: attainthe same potential as the surface of the diaphragm facing it, andtherefore theacoustic output of the diaphragm would consist onlyof thesecond harmonic of the applied A. C. electrical signal; On the otherhand, if the leakage resistancewere very low, the D. C. currents-whichcouldflow betweendiaphragm andthe conductor ofthe stationary electrodewould be of amagni tude suflicient to causeinjurytocomponents of theloudspeaker. Therefore, the sheathing isone that has (1)highdielectric-constant, (2) moderately high leakage resistance, and 3highdielectric strength.

Forthe conditions above described the polariz ing potential divides insuch a manner that a substantially Q nstantpolarizing voltageappearsacross theaingap. In other words; an increaseintotal polarizing"potential betweenterminals increases the-ionization current anddecreases the resistance of the air gap to such an extentthata theactual potential across the air gap is not materially changed,- whilethepotential across the dielectric sheaths isincreased.- It has-beenfoundpossible. in aspeakerof the dimensions; indicated above tomaintain asubstantially 0011- stant air. gap polarizing voltage with a totalpolarizing-- potential anywhere inthe range ofabout 3500 to: 6800 volts.Insuch a case the actual voltage-across-the gap itseh is about 2100-volts. Thiscorrespendstoa voltage gradientof about 50,000 voltsper inchwhich is substantially in-t-he ionization range. The large electrostaticforces-which resultfrom the application ofsuch'a' high electric fieldtogether with the large separation between electrodes rnakeittpossibletool),- tain high acoustic, power output. per unit, area. over theentire range of audio-frequencies. By large en at on'l'" er to. aseuarati nsnmicient to permit the largediaphragm excursionsrreguired,for 0 r q ener nm n tmn, .atthe high e er y s. w h a ob ai i b1.ev bymain aining. the; air gap gradient in or nearthe ionization range.

It has b en. stat d. above: that. hevv deg atin snr ferably not appl ed.uniformly erhe u facehut in areas whereby. th dianhr inl s dirvidedinto. areas moreon ess ele tively, l ssen:- siv'e to certain frequencybands. The areas are preferably of. g neral an ular shape.- l usaasshown n F g, 1) the e is asmell C ntral coated area; W n heform-of.asqnarer.asec ndarea.2&1. urtounding he cent al a ea, andanoutcr area; lemimdthe entiremers ni. The-areas. lit-land; 2.9.; ressgetetesii by; anmn atedzpertion; mend;

the areas 20 and 22 are separated by an uncoated area 26. By this meansthe effective radiating area of the diaphragm may be made a function offrequency, since the various areas may be formed as portions of anelectrical network. Thus, a direct lead 28 is preferably run to theinner area [8, a second lead is run to the intermediate section 20through an impedance 3%, and a third lead is connected to the outer leadthrough .an impedance 32. The impedances 30 and 32 may be pureresistances, and may be constituted, in part at least, by the resistanceof the dag coating.

Since the dag coating does not have a high current-carrying capacity, astripe of conducting silver paste is preferably coated on each dagcoatedarea as indicated at 34, 36, and 38, in order to conduct the currentwith a better distribution to each of the rings.

A network showing diagrammatically the effect of the division of area ispresented in Fig. 6, wherein C1 represents the capacitance of thecentral portion l8 with respect to the back electrode and C2 and 03represent the capacitances of the rings 2!] and 22 with respect to theback electrode. The resistances 3B and 32 are in series with C2 and C3respectively. This network diagram'is not to be confused with Fig. 5since Fig. 5 represents the equivalent network in section, that is,across any portion of the speaker, whereas Fig. 6 represents thedistribution of the resistance and capacitance over the surface. Asillustrated by Fig. 6 the higher frequencies are largely limited to thecentral portion I8, while the intermediate frequencies are restricted tothe central portion l8 and ring 20, and the low frequencies are radiatedby the entire area. This has a two-fold result: first, the impedancelooking into the speaker is more nearly constant with frequency, andsecond, the smaller radiating area for the high frequencies results in alower directivity. In respect to the latter, it is apparent that if thehigh frequencies were allowed to radiate from the entire surface, thespeaker would be highly directional. The smaller radiating area affordedby the central portion for the high frequencies spreads the angularpattern into a satisfactory angle.

The electrical division of the diaphragm into a central zone and annuliis not essential, since the division may be effected in the form ofbands, or as square, rectangles, circles, etc., but preferably inregular geometrical shapes. A segmentation of the type shown in Fig. 8is satisfactory and has the advantage of permitting connections to bemade to the sides of the segments. In this form the corner segment 40 isthe radiating segment for the high frequencies, while the segments 42and 44, which partially surround the corner segment, are addedelectrically at the lower frequencies. Network connections are made asin Fig. 1.

The impedance characteristic of the speaker is illustrated in a generalway in Fig. 4, wherein the logarithm of the impedance is plotted againstthe logarithm of frequency. If it were not for the electrical divisionof the speaker into different zones, the theoretical impedance frequencycharacteristic would be a straight line, indicated by the dash line.However, by reason of the characteristics of the network, thehigh-frequency impedance is increased, and the characteristic assumesthe full line position shown in Fig. 4. The diaphragm may beelectrically divided into any number of areas and the networkcharacteristics may bedetermined according to familiar principles ofnetwork theory, whereby widely varying impedance characteristics may beobtained. It has, however, been found that the use of three zones asshown in Fig. 1 is usually.

adequate for satisfactory reproduction.

An alternative network for the speaker is shown in Fig. 8, wherein the.resistances 30 and 32 are connected to form a ladder network, by which asimilar impedance characteristic may be obtained.

A modified, and in some respects preferable, form of the invention isshown in Fig. 9. This form of the invention acts to maintain asubstantially constant pressure response while providing effectivenon-directional action at high frequencies. In general, this result isattained by varying the acoustic power output per unit area of thediaphragm, preferably by slightly dishing either the diaphragm or theback plate, so that the high frequency segment has a closer spacing'thanthe outer annuli. This arrangement is analogous to the type ofsegmentation shown in Fig. 1. For an arrangement analogous to thesegmentation of Fig. 7 it is only necessary to use a flat diaphragm anda flat back plate which are angled to be relatively close together atthe upper left corner and farther apart elsewhere.

In Fig. 9, the lateral spacing between diaphragm supports preferablyvaries with separation. Accordingly the spacers 5B in Fig. 9 have acloser lateral spacing in the central portions than at outerextremities. Thus the lower frequencies are excluded by mechanicalfiltering action, from the central portion of the diaphragm.

The frequency distribution over the surface of the diaphragm may also beaccomplished by reducing the acoustic transparency of the back plate inselected areas, as by placing the wires 6 very close together at one ormore portions; this has the effect of increasing the high frequencyresponse relative to the low frequencies in the areas of reducedtransparency, and gives a frequency-selective network, in which accountis taken of the effects of mechanical stiffness which is added by theentrapped air.

The speaker may be of any desired size, and is preferably operated at ahigh polarizing potential, which may be about 3500 volts. The frequencyresponse characteristic i highly satisfactory as compared withconventional speakers since it presents fewer sharp peaks and henceresults in an improvement in respect to phase distortion,

While the preferred form of the invention has been described it will beunderstood that it may be varied in respect to detail. The principalfeatures of the invention are, first, the acoustically transparent backelectrode which is insulated in such a way as to permit the operation ofa large air gap at or near the ionization potential of the air while atthe same time it permits the appearance of a high percentage of the A.C. signal voltage across the air gap; second, the me chanical divisionof the diaphragm into irregular and non-symmetrical areas of non-uniformsize for the spreading of resonant frequencies; and third, theelectrical division of the diaphragm into zones or areas whereby thehigh frequency impedance is increased and the directive characteristicsof the speaker are improved.

Having thus described my invention, I claim:

1. A loudspeaker comprising a stationary electrode, a thin diaphragmseparated from the stationary electrode by an air gap, dielectricmaterial completely enclosin the, stationary electrode. whilemaintaining it.acoustically.transparentand having part thereof. betweenthe stationary 91GB: trode and the air gap, and means for applying apolarizing voltage between the stationaryelec: trode and the diaphragm,the dielectric material having a high dielectric constant and a.moderate leakage resistance, the polarizing potential being sufficientto maintain the air gap at a po, tential which is at or near theionizing potential of the gas, the greater part of the signal volt ageappearingacross theair gap;

2. A loudspeaker comprising a stationary electrode of mesh-likestructure whichis acoustically transparent, a thin diaphragm, spacersforsepj arating the diaphragm from the stationary electrode by an air gap,dielectric material completely enclosing the mesh structureand havingpart thereof between the stationary electrode and the air gap, and meansfor applying a polarizing-voltage between the stationary electrodeandthediaphragm, the dielectric material having a high dielectric constant anda moderateleakage resistance, the polarizing potential being sufficientto maintain the air gap at a potential which is at or near the ionizingpotential of the gas, the greater part of the signal voltage appearingacross the air gap.

3. A loudspeaker comprising a stationary electrode consisting of anumber of closely spaced but separated wires, a diaphragm separated fromthe stationary electrode by an air gap, sheaths of dielectric materialcompletely enclosingthe individual wires without filling the spaces be.-tween them, and means for applying a polarizing voltage betweenthestationaryelectrode and the diaphragm, the dielectric materialhavinga high dielectric constant and a moderate leakage resistance, thepolarizing potential being sum? cient to maintain the airgap'at apotential-which is at or near the ionizing potential of the gas, thegreater part of the signal voltage appearingacross the airgap.

4. A loudspeaker comprising a stationaryelectrode, a thin diaphragmseparated-from thestationary electrode by an airgap, dielectric materialcompletely enclosing the stationary electrode while maintaining itacoustically transparent and having part thereof between the stationaryelectrocle and the air gap, and means for'applying' a polarizing Voltagebetween thestationary elec-- trode and the diaphragm, the dielectricmaterial having a high dielectric constant and a -moderate leakageresistance, whereby the airgap may be maintained under a polarizingpotential in the neighborhood of the ionizing potential and the greaterpart of the signal voltage appears across the air gap, and means forestablishing a; non-uniform frequency distribution overthediaphragm..

5. A, loudspeaker comprising a stationary .elec: trade, a thin diaphragmseparated from the sta: tionary electrode, by an air gap, dielectricmate-.- rial. completely enclosing the stationary electrode whilemaintaining it acoustically transparent and having part, thereof betweenthe stationary elec-.- trode and the air gap, and means for applying a,polarizing voltage between the stationary electrode and the diaphragm,the dielectric material having: a high dielectric constant and amoderate leakage resistance, whereby the air gap may be maintained undera polarizing potential in the neighborhood of the ionizing potential andthe greater part of the signal voltage appears across the air gap, andmeans for confining high frequenciesto portions of the diaphragm.

6. A loudspeaker comprising a stationary electrode, a thin diaphragm,and a plurality of spacing means for separating the stationary electrodeand the diaphragm to form an air gap between them, said spacing meansbeing arranged to provide less separation over some portions of thetotal area than over the remainder of the diaphragm and having closerlateral spacing in the region of less separation, whereby some portionshaving a small air gaphave a relatively small free diaphragm area andother portions have a wider air gap and a correspondingly larger freediaphragm area.

7'. A loudspeaker comprising a stationary electrode,v a thin diaphragm,a conducting coating for the diaphragm, the diaphragm being electricallydivided into a small zone and at least one larger zone, network meansfor substantially confining high frequencies to the small zone, andspacing means for separating the-stationary elec-- trode and thediaphragm, said spacing means being variable to provide a smallerseparation in the high frequency zone than elsewhere, said spacingmeansalso having closer lateral spacing in the high frequency zone.

A. JAN SZEN.

GES CIT ED The following references are of record in the file of thispatent:

D A E AT N S

